1. Technical Field
The present invention relates to a processing method for strengthening materials such as thin plates and rods made of carbon steel, and specifically relates to a strengthening process for improving fatigue strength.
2. Background Arts
Until recently, it had been believed that the fatigue strength of mechanical parts, specifically in carbon steel materials used for parts in automobiles, among mechanical properties of the materials, greatly depends on tensile strength. Therefore, fatigue limits such as the number of repeated rotating and bending cycles, the number of repeated tensioning and compressing cycles, the number of repeated twisting cycles, and the like have been evaluated based on tensile strength. However, it is well seen that even if tensile strength is improved, fatigue strength cannot be easily improved, or there may be a region with no improvement in fatigue strength. Therefore, in order to improve fatigue strength, methods for hardening surfaces of materials by providing residual compressive stress thereto through several types of plastic processing, such as rolling, form rolling, drawing, and shot-peening have been generally applied.
However, in a method such as the above in which fatigue strength is improved, although fatigue properties of a material can be somewhat improved, fatigue strength of the overall material could not be improved. As a result, a portion with low fatigue strength may be the initiation site of a fracture, and this readily results in premature failure. Furthermore, in a shot-peening processing, a number of conditions such as shot diameter, incident speed, frequency of shot-peening processing (number of steps), and shot washing vary for each type of material, so that extensive experimentation was required to determine the optimal conditions for a specific case. The compressing processing such as rolling, form rolling, and drawing similarly require extensive experimentation to select processing conditions.